1. Field of the Invention
The present invention relates to a strain wave gearing including an electric motor and a strain wave gearing reducer for reducing a rotational speed of the electric motor, and also relates to a robotic arm including the strain wave gearing.
2. Description of the Related Art
There is known a strain wave gearing including an electric motor and a strain wave gearing reducer (so-called harmonic drive (registered trademark)). The strain wave gearing of this type is provided to each joint of links of a robotic arm so as to pivot and rotate the links. The strain wave gearing reducer includes a thin, cylindrical, flexible external gear having external teeth and called flex spline, an internal gear having internal teeth and called circular spline, and a cam member called wave generator (see Japanese Patent Application Laid-Open No. S63-053340, Japanese Utility Model Publication No. H04-048346, and Japanese Utility Model Application Laid-Open No. S61-011047). The wave generator is formed into an elliptical shape so that the flex spline is deformed into an elliptical shape and pressed against the circular spline.
FIGS. 13A, 13B, and 13C are sectional views of three strain wave gearing reducers. First, one of the strain wave gearing reducers is described with reference to FIG. 13A. A stator 101 of an electric motor is provided to a fixed shaft 100, and a rotor 102 is arranged along an outer periphery of the stator 101. A wave generator 103 is fixed to the rotor 102. Along an outer periphery of the wave generator 103, an open end of a flex spline 104 is mounted, and the other end of the flex spline 104 is supported so as to rotate freely relative to the fixed shaft 100. Along an outer periphery of the flex spline 104, a circular spline 105 fixed to a cover 106 is arranged. The flex spline 104 is distorted by the wave generator 103 into an elliptical shape to engage with the circular spline 105 at two positions, that is, both ends along the major axis of the ellipse. The rotation of the wave generator 103 constructed by the rotor 102 of the electric motor causes a relative rotation between the flex spline 104 and the circular spline 105, and a rotation output is extracted by a flange 107 fixed to one end of the flex spline 104.
Next, another strain wave gearing reducer is described with reference to FIG. 13B. The strain wave gearing reducer includes two circular splines 111 and 112 having different numbers of teeth. A flex spline 113 is provided on an inner side of the two circular splines 111 and 112, and there is provided a wave generator 115 for distorting and deforming the flex spline 113 into an elliptical shape so that the flex spline 113 is rotated in the elliptical shape. The rotation of the wave generator 115 causes a change in engagement positions between the flex spline 113 and the two circular splines 111 and 112. When the engagement positions change during one revolution, the circular spline 111 rotates relative to the circular spline 112 with the shift corresponding to the difference in number of teeth between the two circular splines 111 and 112. In this strain wave gearing reducer, one of the two circular splines 111 and 112 is used as a fixed shaft while the other is used as an output shaft.
Next, still another strain wave gearing reducer is described with reference to FIG. 13C. The strain wave gearing reducer includes one elliptical wave generator 121 fixed to an input shaft 120, and one flex spline 122 having a tooth form on an outer periphery thereof. The strain wave gearing reducer further includes a first circular spline 123, which is fixed to an arm 123a provided in a part of an outer periphery thereof and engages with the flex spline 122, and a pair of second circular splines 125 and 126, which engage with the flex spline 122 on both sides of the first circular spline 123. In this strain wave gearing reducer, the pair of second circular splines 125 and 126 are coupled to each other by a coupling bar 124, and the pair of second circular splines 125 and 126 rotate relative to the first circular spline 123. The coupling bar 124 is used as an output shaft.
An industrial robotic arm is structured by connecting multiple joints in series. In each joint, the strain wave gearing including the electric motor and the strain wave gearing reducer is disposed, and hence the mechanical model is a model in which stiffnesses of the respective strain wave gearing reducers are connected in series. Therefore, unless a strain wave gearing reducer having a sufficiently high stiffness is used, the stiffness of the entire robotic arm lacks. If the stiffness of the robotic arm is low, the natural frequency of the robotic arm decreases, resulting in decrease in accuracy, increase in stabilization period, decrease in maximum operation speed, and other such performance degradation. The use of the strain wave gearing reducer having a high stiffness is an important factor for the robotic arm.
In the strain wave gearing reducer illustrated in FIG. 13A, a rotational force applied to a portion between the circular spline 105 and the flange 107 is also applied to the flex spline 104 provided therebetween. When the rotational force is applied to the flex spline 104, the flex spline 104 being a flexible elastic member is torsionally deformed by the rotational force, and the flange 107 serving as the output shaft rotates by the torsional deformation. In the strain wave gearing reducer illustrated in FIG. 13A, the flex spline 104 serves as the output shaft, and hence the flex spline 104 is likely to be torsionally deformed, which causes the decrease in stiffness of the strain wave gearing reducer.
In the strain wave gearing reducer illustrated in FIG. 13B, the circular spline 112 situated in one end portion of the flex spline 113 rotates relative to the circular spline 111 situated in the other end portion. Hence, forces are applied to the external teeth of the flex spline 113 in opposite directions in a part engaging with the circular spline 111 and in a part engaging with the circular spline 112. Because the forces are applied to the respective end portions of the flex spline 113 in opposite directions, the flex spline 113 is likely to be torsionally deformed by the forces, which causes the decrease in stiffness of the strain wave gearing reducer.
In the strain wave gearing reducer illustrated in FIG. 13C, the two circular splines 125 and 126 situated in both end portions of the flex spline 122 rotate relative to the circular spline 123 situated in the center portion of the flex spline 122. The two circular splines 125 and 126 rotate in the same direction, and hence forces are applied to the end portions of the flex spline 122 in the same direction. Accordingly, the direction of the force applied to the center portion of the flex spline 122 is opposite to that of the force applied to each end portion, but the directions of the forces applied to both the end portions are the same. Thus, the flex spline 122 is not likely to be torsionally deformed, which suppresses the decrease in stiffness of the strain wave gearing reducer. In this strain wave gearing reducer, the two circular splines 125 and 126 are coupled to each other by the coupling bar 124 extending outside. Hence, the coupling bar 124 hits against the arm 123a supporting the circular spline 123, and accordingly no further revolution can be made than one revolution. When the strain wave gearing including the strain wave gearing reducer having such structure is used for the robotic arm, the movable range of the links is reduced.